5 Real DIY Aerodynamic Mods Detroit Can Add for MPGs in '09

You don't need a wind tunnelor even a welding kit, reallyto know which way the wheels go. PM's guest hypermiler tracks down five average joes whose simple, cheap DIY car adjustments are maxing out fuel efficiencyand could inspire the world's top automakers.

It's an accepted notion that cars roll off the assembly line tuned perfectly for every condition. In reality, automakers need to meet specifications for a variety of parameters--ground clearance, crash testing, fuel efficiency, ride and handling, design, NVH (noise, vibration and harshness) and aerodynamics, to name a few. And to meet all these demands, cars are chock-full of compromises. For over 60 years, hot rodders (or "tuners," as they're now known) have modified everything from a '32 Ford to a '92 Civic for better performance. Gear heads take stock rides and push the speed envelop as far as it will go--and sometimes beyond--to cut a quicker ET at the dragstrip or a quicker lap time around a road course.

But with gas soaring past $4 a gallon in the United States, a new type of hot rodding has taken hold with the growing cognoscenti of aeromodders. Instead of tuning for quicker quarter-mile time slips, these modders alter the vehicle's aerodynamics in order to reduce drag and increase fuel economy. It's a relatively easy way to add passive fuel-sipping gains to just about any car. And after seeing so much of this tricked-out DIY trend in the growing library of hypermiling tips across the Web and on the forums at my Ecomodder online community, I wonder if the big OEMs couldn't learn a thing or two.

Vehicles are designed to meet the demands of everyday life in an amazingly complex variety of conditions, from a scathing-hot Death Valley dirt road to a bone-chilling Alaskan two-laner in the dead of winter. But in times like these, backyard modders are willing to trade some of that varied-terrain capability for increased efficiency. Ecomodders can look at a car and see right away what needs to be improved to maximize fuel economy, whether it's a rough underbelly, uncovered rear-wheel wells or a gaping grille.

These mods are inexpensive to make. And just about any mileage enthusiast can potentially do it, since ecomodding doesn't really require hardcore mechanical skills. Here's a look at some DIY tricks from real guys around the country who are pushing the envelope on aeroefficiency--and bumping up the miles per gallon.

Grille Aero Treatment

The Modder /// Andrew Johnmeyer is interested in going extreme to improve the aerodynamics of his Civic.

How It Works /// Johnmeyer lowers the grille to prevent air from flowing under the car, where the typically rough surface can impede airflow. His block also smoothes out the front of the car and prevents air from entering the grille. Because blocking off the grille makes it harder for the radiator to cool the engine, he has cut a small vent that he can open or close from inside the cabin if he needs to. Grille blocks are not usually needed during the winter, when full radiator exposure is often overkill, but in stop-and-go traffic and during the summer, it's good to be able to add airflow to the radiator on demand. The block works because air entering the radiator and engine bay is often one of the largest contributors to aerodynamic drag in modern cars.

How the Automakers Can Do It /// Honda has experimented with grille openings that close at high speeds, while the new Mercedes BlueEfficiency package includes smaller grille openings.

Truck Bed Cap

The Modder /// Brett Herndon is looking to market his aero cap as an aftermarket upgrade for truck owners.

How It Works /// There's plenty of debate among truck guys about whether it's more efficient to drive with the tailgate up or down, or with or without a truck-bed cap. Aero caps make short work of that question. They're more efficient than either of those options: They create an efficient, teardrop-like shape, greatly improving aerodynamics. By tapering toward the rear of the vehicle and minimizing the size of the airflow's wake, Herndon's cap increases his fuel economy from 18 mpg to 22 mpg on the highway. That 20 percent improvement may not seem like a lot, but over a relatively short time, it would pay for the cap--and then some.

How the Automakers Can Do It /// The great thing about aero caps is that they function just like normal bed covers: They can be added and removed as needed, and can be sold as accessories for trucks without obligating every owner to use aero styling all the time.

Boattail

How It Works /// Perhaps counterintuitively, a large portion of aerodynamic drag has to do with how air leaves the car. That is, the bigger the wake size, the more air that's disturbed behind the car and the more energy it takes to move the car. The solution to this problem is either to have a smaller car with a smaller overall footprint, or to have a car that tapers toward the end of its body. A taper that's small enough to prevent the air from becoming turbulent but large enough to affect the size of the wake greatly improves aerodynamics. Like the truck aero cap, most boattails seek to transform the rear end of the car into a more teardrop-like shape.

How the Automakers Can Do It /// It's not likely that we'll be seeing radical boattails from Detroit anytime soon, but as aerodynamics comes back into focus, we expect to see more fastbacks and more attention paid to smoothly tapered bodywork.

Wheel Skirts

The Modder /// Darin Cosgrove, owner of MetroMPG and co-founder of EcoModder.

How It Works /// Air passing down the side of the car traditionally hits the front and rear wheel well on the way to the back of the car. The wheel wells are a source of turbulent flow, and though it's not easy to do anything about the front wheel well (because the wheels have to turn), the rear wheels can easily be covered up flush with the body of the car in order to streamline the car. Wheel skirts promote a smoother air flow so they benefit the overall aerodynamics.

How the Automakers Can Do It /// Both the Honda Insight and GM's doomed EV1 electric car had aerodynamic rear wheel skirts. In fact, the EV1 used flexible membranes to add front wheel skirts, as well.

Undertray

The Modder /// Lunarhighway is an Ecomodder member from Belgium trying to avoid gas prices of nearly $6 a gallon.

How It Works /// The underside of your car is one of the most aerodynamically rough parts of the car. However, it doesn't have to be that way. On most cars, a sheet of aluminum or corrugated plastic can easily cover the entire underside, greatly reducing drag. Oftentimes, because of the relatively consistent ground clearance of the car, it is enough to build front and rear partial undertrays. Though not as beneficial, this method ensures that smooth airflow begins at the front of the car and then is not caught by the parachute-like bumper at the end.

How the Automakers Can Do It /// Many companies are experimenting with partial undertrays; they are seen as an easy way to improve aerodynamics without changing the look of a car.